八卦五行 - The Eight Trigrams & Five Elements

八卦指代八个方向，所以八卦和八方五行一一对应。

由此得到助记表格，

方位	先天八卦象	后天八卦象	五行含义
东	地	雷	阳木（树木）
东南	雷	风	阴木（花草）
南	火	火	火
西南	泽	地	阳土（山石）
西	天	泽	阳金（金属刀兵）
西北	风	天	阴金（珍宝矿藏）
北	水	水	水
东北	山	山	阴土（田园泥土）

由于上古中古与近古出现的历史断代，怀疑其中经历过一次地球磁极自东西转南北的巨大改变。从中西方神话、历史来看，有过一次“大洪水”时代似乎可以印证。在中华大地上，差不多是“大禹治水”的时间（也可能是更早的“共工触不周山女娲补天”那个时间）。

伏羲受河图启发创造先天八卦。河图早于这个大洪水时代，体现的很有可能是东西磁极的情况。因此从图像上来看，先天八卦是“左南右北上西下东”的样子。先天卦象指代的五行可能也有断层，因此在《周易》里涉及到的八卦五行含义是保持方位一致的。

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"Eight Trigrams (Ba Gua)" refers to eight directions, so each of the eight trigrams corresponds to one of the five elements.

From this, a mnemonic table is obtained,

Direction	Primordial Trigram	Derived Trigram	Meaning of Five Elements
East	Earth	Thunder	Yang Wood (Trees)
Southeast	Thunder	Wind	Yin Wood (Grass)
South	Fire	Fire	Fire
Southwest	River	Earth	Yang Earth (Mountains and rocks)
West	Sky	River	Yang Metal (Metal knives and weapons)
Northwest	Wind	Sky	Yin Metal (Precious metals and minerals)
North	Water	Water	Water
Northeast	Mountain	Mountain	Yin Earth (Farmland and soil)

It is believed that there might have been a historical discontinuity, a great change in which the Earth's magnetic pole shifted from east to west and north to south. This change might have resulted in a "great flood" era, which is reflected in ancient Chinese mythology.

The ancient Chinese sage Fuxi was inspired by the Lo Shu Square to create the "Primordial Trigram" which was created before this great flood era and reflects the situation of the East-West magnetic pole, therefore, the image of the "Primordial Trigram" is "left south, right north, top west, bottom east." The five elements of the trigrams may also have been affected by this discontinuity, so in the "I Ching" (Book of Changes) the meanings of the eight trigrams are consistent with the directions.